Was in Camden Town the other day with some time to waste. Ended up looking around a quite well stocked ironmongers that had a lot of good paint brushes hanging up.
There were a few brushes there in the normal kind of widths, but were quite thin (about half the thickness of a usual bristle brush).
The assistant said that they were varnish brushes. Looking back on it i cannot think why varnish brushes would be thinner. Can anyone say why please?